Unless someone has lived 24/7 with a person suffering from dementia they have no idea what it's like. As Oldnproud says, she is not likely to be young herself, so the stress of looking after a person with dementia could be affecting her in ways that you and she might not be aware of.
A carer may be determined to look after the person themselves, without outside help, but they can melt down too, at the drop of a hat. What seems obvious to you - accepting help for instance - might be something she just can't accept. "Common sense" goes out the window when you are faced with your life partner behaving in ways that you don't want to see, and demanding things you can't provide.
Speak to social services, adult protection, NHS, everybody you can and keep pushing. They will try to push it all on to families/partners when what your DF may need is full time residential care, or at least visiting carers. There is a time when your DF will become so incapacitated by his illness that a care package will be more than recommended, it will become obligatory.
As for yourself, don't feel guilty about not visiting as often as your DF and his partner want you to. The demands of people with this illness can become infinite and you have a life to lead too.